The present invention relates to threaded fasteners and in particular to a self-tapping screw for use in plastics.
There currently exists a number of threaded fastener products which are specifically adapted to use with plastics. The primary consideration in the design of such threaded fasteners is to balance the failure load of the metal screw thread to that of the plastic thread formed. The most significant development toward this end has been to widen the space between the threads and also to make the threads as deep as possible to provide a greater amount of plastic between the threads.
One of the drawbacks with this approach, however, is that when torque or tension is applied to the screw, the plastic material between the screw threads tends to move down the thread flank away from the axis of the screw, thus compressing the plastic outwardly causing premature stripping of the screw thread.
Accordingly, it is the primary purpose of the present invention to provide a self-tapping threaded fastener for use with plastic material that minimizes the problem of premature stripping by eliminating the outward compression of plastic material away from the threads. In particular, the self-tapping threaded fastener according to the present invention has a negative rake or "hook" formed on the top flank or crest of the threads so that when torque or tension is applied to the screw, the plastic material between the threads is forced inwardly toward the axis of the screw. Thus, the torque or tension load required to cause the screw assembly to fail is significantly increased.
Also disclosed herein is the preferred method for making a threaded fastener according to the present invention. More particularly, a two-part rolling die is used having a first part that produces the basic thread pattern and a second part that bends the crests of the threads upwardly towards the head of the screw to create the desired "hook" effect on the top flanks of the threads.
Additional objects and advantages of the present invention will become apparent from a reading of the detailed description of the preferred embodiment which makes reference to the following set of drawings in which :